One of the most effective media for treating a gas such as an internal combustion engine exhaust gas to remove solid particulate matter therefrom, comprises shredded metal, such as steel wool or the like. When properly shaped, such a media, even though frangible in nature, can be compressed to give a desired density and filtering capacity for contacting gas passing therethrough.
In many instances, filters or reactor beds of this type are coated with an external material to achieve a particular result. The coating can comprise a catalyst or other suitable element which is capable of promoting a reaction when contacted by a hot gas.
In the instance of the treating of an exhaust gas from an internal combustion engine, the gas often reaches temperatures having an order of magnitude of 1600.degree. F. Such elevated temperatures are deleterious to a metallic filter of the type contemplated since they can substantially weaken or thermally degrade the metal. Thus, over a period of time, the entire filter or reactor bed will tend to distort or even collapse under gas pressure.
When the latter occurs, the bed will shrink and draw away from its enclosing casing. Such an action creates bypasses for the flow of gas such that at least a part of the latter can avoid the filter media and pass directly to the filter discharge port.
It is of further note that filters or reactor beds of the type contemplated are normally initially assembled with the bed inside a casing. The assembled unit is thereafter treated with a catalyst or similar coating. The surfaces of the filter bed will not under such circumstances receive a homogeneous coating.
For example, those parts of the bed which contact casing walls can be deprived of a coating layer or receive but a minimal layer. In effect, the active coating will not only be incomplete; it will often be lacking at the periphery of the filter bed. This segment of the bed can be vital, particularly when and if the bed tends to contract and draw away from adjacent casing walls as a result of exposure to the gas being treated.
In overcoming the above stated problems, there is presently provided a method for forming a filter or reactor bed particularly adapted for treating a stream of particles carrying hot exhaust gas.
The unit embodies essentially a reactor bed which is formed of a media comprised of randomly disposed fibrous metallic strips. The latter are preferably formed of a metal such as steel wool or the like. Prior to being assembled into a gas confining casing, the bed is preformed to provide it with a desired size, density and shape to best register within a particular casing.
While being retained in the desired shape within retaining means, the media bed is treated with a coating solution such that all exposed surfaces receive a preliminary hardenable layer. Thereafter the bed is removed from its restraints. However, the thin preliminary layer is sufficiently supportive to maintain the metallic fibers making up the bed within the latter's required dimensions. The partially coated, though still fragile form is now immersed into the same or other coating solution wherein it is maintained until such time as the required thickness or layer has been deposited onto the preliminary layer.
It is therefore an object of the invention to provide a method for fabricating a unit which is capable of removing solid particles from a hot exhaust gas stream. A further object is to provide a method for fabricating a filter or reactor of the type contemplated which includes a preformed bed adapted to receive a stream of gas to be treated which bed is characterized by a homogeneity of structure and density. Another object of the invention is to provide a method for applying a substantially uniform layer of a coating material onto the diverse surfaces of a filter or reactor bed comprised of randomly disposed metallic fibers or strips. Still another object is to provide a method for premolding and treating a fibrous mass of reactor media to apply a uniform coating to all the surfaces thereof.